1. Field of the Invention
The present invention relates to methods of and apparatus for generating a vapor for heating articles, and in particular to a method of and an apparatus for generating a vapor for heating an article by boiling a liquid which is floating on a pool of molten metal.
2. Description of the Prior Art
In soldering, fusing or brazing an article, it is necessary to heat the article to elevate its temperature in order to perform the operation. Although the present invention for heating articles is not to be construed as limited to soldering, its background is most readily understood in the context of soldering.
Soldering operations require that the elements to be soldered be heated to a temperature sufficiently high to melt solder thereon, which elements are preferably protected from oxidation during the soldering operation. Conventionally, a soldering iron is used, and the elements are coated with a flux on those areas to be soldered to prevent oxidation thereof during soldering. While the soldering iron is useful in many instances, it is not well suited to rapid mass soldering operations in which, for example, a printed circuit board or terminal array may have many closely spaced connections to be soldered.
One prior art improvement over the soldering iron technique for use in mass soldering operations is disclosed in U.S. Pat. No. 3,359,132, which issued on Dec. 19, 1967, to A. E. Wittmann. In this patent a circuit board, which has a coating of flux, is suspended in close proximity to the surface of a hot layer of peanut oil, which is floated on a pool of molten solder, for being preheated by a combination of radiation and air convection. The circuit board is then lowered through the layer of peanut oil and into the molten solder wherein the soldering operation takes place. This technique has the disadvantage of requiring the cleansing of the circuit board to remove the peanut oil therefrom after the circuit board is again passed through the layer of peanut oil upon its removal from the molten solder.
Recently, a new soldering technique was developed and placed in commercial use. With this technique, which is the subject of U.S. Pat. No. 3,866,307 issued Feb. 18, 1975, and assigned to the assignee of the present invention, the article to be soldered (or fused or brazed) is placed in hot saturated vapors generated by continuously boiling within a vessel a heat transfer liquid having selected properties including a boiling point at least equal to, and preferably above, the temperature required for the soldering operation. Vapors condense on the article and give up their latent heating of vaporization to heat the article to the temperature required for soldering. In one specific application, the process is employed to effect mass reflow soldering.
The soldering technique described in the preceding paragraph represents, in the opinion of those familiar with soldering, fusing or brazing operations, a state of the art advance. In the practice of such technique, however, some of the saturated vapors of the heat transfer liquid escape from the vessel and are lost to the atmosphere where the process is open to the atmosphere. The heat transfer liquid presently employed in the practice of the preferred embodiment of the technique is a florinated polyoxypropylene, such as that sold by E. I. DuPont de Nemours and Company under the name "Freon E5," and under presently prevailing price schedules is quite expensive. Consequently, losses of the vapors to the atmosphere represent an economic charge against the technique as presently practiced, and it is desirable that these losses be reduced or eliminated.
To overcome the loss of "Freon E5" to the atmosphere, an improvement in the above technique was developed, and is the subject matter of pending U.S. patent application Ser. No. 476,343, filed June 5, 1974, now U.S. Pat. No. 3,904,102, and also assigned to the assignee of the present invention. In this improved technique, heated vapors of a relatively inexpensive secondary liquid are interposed between the heated vapors of the "Freon E5," hereinafter referred to as the primary liquid, and the atmosphere. This is accomplished by boiling a mixture of the primary and the secondary liquids in the lower portion of a vessel to generate hot saturated primary and secondary vapors thereof in the upper portion of the vessel. The secondary liquid is chosen such that the boiling point thereof is lower than the boiling point of the primary liquid, and such that the secondary vapors generated upon the boiling thereof have a density less than the density of the primary vapors generated upon the boiling of the primary liquid, and greater than the density of air at atmospheric pressure. In this manner, as both of the liquids are boiled, the secondary vapors form in the vessel a barrier between the primary vapors and the atmosphere to prevent, or minimize, the loss of primary vapors to the atmosphere. An article to be soldered in accordance with this technique is extended through the secondary vapors and into the primary vapors for being heated to the soldering temperature as the vapors condense thereon, and is then removed through the layer of the secondary vapors for cooling in the atmosphere.
In the practice of either of the above two soldering techniques, with or without the secondary vapor intermediate the primary vapor and the atmosphere, the liquid is customarily heated in the bottom of the vessel by an immersion heater to generate the vapor thereabove. When this is the case, the immersion heater must be completely surrounded by the liquid at all times to prevent its destruction as a result of superheating. Where the secondary vapor is not generated, the liquid surrounding the immersion heater is purely primary liquid, and where the secondary vapor is generated, the relative quantities of primary and secondary liquids is such that the liquid surrounding the immersion heater is essentially primary liquid.
The minimum volume of primary liquid required in a vessel, where an immersion heater is used, is mainly determined by the weight of the article to be heated. That is, the weight of the article determines the power required to heat the primary liquid, which in turn determines the size of the immersion heating element, or the number of immersion heating elements, required in the bottom of the vessel, which then in turn determines the minimum volume of primary liquid required in the vessel to ensure that the heating element is completely covered. In other words, a sufficient volume of primary liquid must be maintained in the vessel to ensure that the heating element remains covered at all times, which volume is much greater than that required for only generating vapors to be condensed upon an article. For example, with one current facility for vapor condensation heating of articles, which employs a 20 watt/cm.sup.2 immersion heating element, approximately 65 gallons of primary liquid are required in a vessel for soldering 75 pound articles. Of the 65 gallons of primary liquid, approximately 55 gallons thereof are employed solely to cover the heating element to prevent the destruction thereof. At the present price of "Freon E5," which is a preferred primary liquid, this represents a considerable additional expense. Furthermore, when the primary liquid surrounds the heating element, it is possible for film boiling thereof to occur, resulting in destruction of the heating element.